Degradable polyester/urea inclusion complex applied as a facile and environment-friendly strategy for slow-release fertilizer: Performance and mechanism

Most manufacturing technologies of slow-release urea fertilizers are solvent-assisted and consequently complicated. Therefore, the development of a solvent-free and facile approach has been actively pursued. Herein, the inclusion complexes (ICs) of environmentally degradable polyester and urea prepared by one-step blending are developed as slow-release urea fertilizers for the first time. Three kinds of polyesters, poly(butylene succinate) (PBS), poly(ethylene succinate) (PES) and poly(propylene succinate) (PPS) are involved. Compared with neat urea, the IC fertilizers exhibit much lower and more adjustable release rates, which can decrease from tens to thousands of times by varying factors such as granular size, compactness and polyester species. Interestingly, the crystallizability of polyester is demonstrated to play an important role in controlling the slow-release performance, the weaker crystallizability of polyester chains results in the longer release time of IC fertilizer. Analysis of release profiles reveals that the urea component in IC fertilizers displays a second-order release process first and then a zero-order behavior at the late stage. Microstructure assay suggests that the insoluble polyester chains freed from urea frameworks crystallize and sheath the rest of IC upon contact with water. Consequently, a sophisticated release model of urea from IC fertilizer is proposed. The degradation of polyester component and the promotion effect of IC fertilizer on maize growth are also studied in the lab.